The present invention relates to a method for producing a catalyst for purification of exhaust gas of an internal combustion engine.
Catalysts for automobiles have a function to decompose and remove hydrocarbons (HCs), nitrogen oxides (NOx), and carbon monoxide (CO), which are harmful components in exhaust gas. Such a catalyst is produced by using, as a substrate, an inorganic material such as cordierite or a metal formed into a honeycomb shape. The harmful components are decomposed and removed when exhaust gas passes through the inside of the catalyst. To increase the contact efficiency of the catalyst with exhaust gas, the surface of the substrate is coated with a porous inorganic material, and furthermore, a trace amount of a noble metal is supported as an active component on a surface layer portion of the porous inorganic material. A platinum group metal such as platinum, palladium, or rhodium is used as the noble metal.
Recently, in order to rapidly treat exhaust gas at the ignition of an automobile engine, there has been a need for activation of the catalyst even at low-temperatures. In this respect, improvement of purification performance (low-temperature activity) has been attempted by increasing the amount of the noble metal. However, the increase in the amount of the expensive noble metal leads to increase in costs of automobiles, and is disadvantageous to consumers. Hence, there has been a need for improvement in purification performances without increase in the amount of the noble metal.
A platinum hydroxide polymer is a hydroxide in which platinum atoms are cross-linked with about several to several tens of oxygen atoms. For preparation of the platinum hydroxide polymer, hydrogen hexahydroxoplatinate (H2Pt(OH)6) is used as a raw material. The platinum hydroxide polymer is formed as follows. Specifically, hydrogen hexahydroxoplatinate is caused to be present in the form of a hydroxo complex by being dissolved in a strong acid solution, and then the complex is subjected to influence of protons from the acid to thereby form reactive monomers and, in turn, to cause a polymerization reaction. Since the polymerization reaction proceeds rapidly, it is difficult to perform control during the reaction. However, when the acid concentration, the reaction temperature, and the raw material concentration are set to satisfy specific conditions, the polymer turns into a metastable state under which the polymer has a degree of polymerization depending on the conditions, and under which the reaction is temporarily stopped. In addition, since the polymerization reaction is irreversible, the polymerization state can be maintained even when the temperature is lowered from the reaction temperature. The number of platinum atoms contained in a polymer varies among platinum hydroxide polymers prepared as described above, depending on the degree of polymerization. Hence, it is possible to use this polymer, for example, as a precursor material for controlling a particle diameter of the noble metal for an exhaust gas catalyst or the like.
However, the platinum hydroxide polymer has a low adsorption ratio on a support made of alumina (Al2O3), a ceria (CeO2)/zirconia (ZrO2)-based composite oxide, or the like. Hence, it is difficult to support a desired amount of platinum by use of the platinum hydroxide polymer. For this reason, it is difficult to use a solution containing the platinum hydroxide polymer as a material for a catalyst produced in a large quantity.
Japanese Patent Application No. 2011-105106 reports a solution containing a platinum hydroxide polymer having a controlled particle size, and a solution with a stabilized polymer size obtained by adding Zr ions to the above solution. When any of these solutions is used as it is, the platinum hydroxide polymer is not readily adsorbed on a support, and only part of the platinum in the solution is adsorbed on the support. For this reason, a platinum-supporting catalyst can be obtained only by a method (water-absorption-supporting method) in which a support is immersed in a solution having a high platinum concentration for several minutes, and the support in which the solution is absorbed is calcined.
However, when platinum is supported by the water-absorption-supporting method, it is difficult to make an amount of water absorbed constant. Moreover, even a slight difference in the amount of water absorbed leads to a large difference in the amount of platinum supported, because a solution having a high platinum concentration is used. In addition, since at least part of the platinum is adsorbed on the support when the support is immersed in the platinum solution, the platinum is supported in a larger amount than an amount of platinum supported which is estimated from the amount of water absorbed. Hence, it is difficult to support a desired amount of platinum by the water-absorption-supporting method, and it is difficult to use a solution containing a platinum hydroxide polymer as a material for a catalyst produced in a large quantity.
Moreover, the platinum hydroxide solution used in the water-absorption-supporting method has a very high acidity with a pH value of 0 or lower. For this reason, when a support in a honeycomb shape is immersed in the platinum hydroxide solution, the platinum hydroxide solution inflicts damage on the support, such as deterioration in the mechanical strength of the substrate due to elution of the cordierite component of the substrate.
On the other hand, when a support made of alumina, a ceria/zirconia-based composite oxide, or the like is immersed in a dilute dinitro diamine platinum solution, which is widely used as a solution for supporting platinum, for a whole day and night, almost the entire amount of platinum in the solution adsorbs on the support. A platinum-supporting catalyst can be obtained by calcining this support (impregnation-supporting method). Hence, the impregnation-supporting method using a dinitro diamine platinum solution makes it possible to obtain a catalyst on which platinum is supported in a desired amount controlled by changing the platinum concentration in the solution, the amount of the solution, and the like, as appropriate.